12 
MESSRS. C. T. HEYCOCK AND F. II. NEVILLE ON 
decomposes into a very minute complex (fig. 19) of a and the tin-rich body S, which 
we strongly suspect is the compound Cu^>Sn. Thus at all temperatures below h'C' 
this group of alloys consists of the complex a -j- 
(3.) The LC alloys, containing from 13‘5 to 15'5 atomic per cents., that is from 
22’5 to 25'5 per cent, by weight of tin. These alloys commence their solidification 
by the formation of a crystals, hut this process soon ceases, for at the C temperature 
the a is wholly changed into /3 of the I percentage, and then this /S reacts along tlie 
lines Ic and CD with the residual licpiid in the manner described on p. 8. When 
the temperature has fallen to the solidus Ic, the alloy is a uniform solid solution. It 
is a mass of ^ crystals, chemically identical, hut forming crystalline grains differently 
oriented and therefore sliowing, after etching, difierences of brightness on tilting or 
rotating. This uniform solid solution continues to exist until the temperature falls 
below the line IC', which is comparable to a freezing-j^oint curve, inasmuch as on 
cooling to a point on this line, the uniform /3 becomes saturated with a, and below the 
line the a crystallises out in large cojrper-rich crystals. Finally, as before at 500°, 
tlie residual /3 Irreaks up into the C' complex. Below the h'C' line these alloys, like 
the preceding, consist of the complex of «-{-§. An examination of the photographs 
we give of Sn 14 (Plate 3) will be found to confirm these statements. The C' complex 
has often, of course, been previously observed in unchilled alloys, but, so far as we 
know, without its real nature Ijeing discovered. 
(4.) The CD alloys, containing from 15'5 to 20 atomic per cents, of tin, that is from 
25’5 to 31’8 per cent, by weight. These alloys begin to solidify by forming large, 
comparatively cop 2 :)er-rich combs of /3 immersed in a licjuid considerably richer in tin, 
and, as far as Sn 16’5, the alloys when just solid, are a uniform mass of /S crystals. 
Although the alloys from Sn IG‘5 to Sn 20 form similar j3 crystals during the first 
stages of solidification, these crystals never entirely fid the ingots, and below the 
temperature D they are transformed into what appears to be uniform y. The uniform 
character of the solidified alloys persists so long as the tem23erature is above the line 
C'XD'. These solid solutions are very homogeneous, and we have not been able to 
detect much, or any, difterence Ijetween the ajopearance of the uniform ^ of Sn IG 
at 700° and the uniform y of Sn 18 at the same temperature, provided both alloys 
have been very slowly cooled. 
In all the alloys of this group, when the tem 2 )erature falls below C'XD', the phase S 
crystallises out in ribands at tlie borders of the crystal gi’ains of tlie solid solution, as 
well as in the form of fern-leaf or rosette scattered through the gi-ains. This 
crystallisation is well seen in the photograjdis of Sn 17, 18, and 19, that accom^Dany 
the paper (see Plates 4 and 5). The S, as seen in the photographs, is white ; it contains 
a larger jirojiortion of tin than the mother-solid round it, and the S increases in amount 
as the percentage of tin in the whole alloy increases. The S first appears in the 
ribands whicli border the grains, tliese ribands being produced at higher temperatures 
tlian the fern-leaf We are disposed to think that this peculiarity is due to the fact 
